An essential role for retinoid receptors RARbeta and RXRgamma in long-term potentiation and depression.

Hippocampal long-term potentiation (LTP) and long-term depression (LTD) are the most widely studied forms of synaptic plasticity thought to underlie spatial learning and memory. We report here that RARbeta deficiency in mice virtually eliminates hippocampal CA1 LTP and LTD. It also results in substantial performance deficits in spatial learning and memory tasks. Surprisingly, RXRgamma null mice exhibit a distinct phenotype in which LTD is lost whereas LTP is normal. Thus, while retinoid receptors contribute to both LTP and LTD, they do so in different ways. These findings not only genetically uncouple LTP and LTD but also reveal a novel and unexpected role for vitamin A in higher cognitive functions.

RXR alpha deficiency confers genetic susceptibility for aortic sac, conotruncal, atrioventricular cushion, and ventricular muscle defects in mice.

Retinoid-dependent pathways play a central role in regulating cardiac morphogenesis. Recently, we characterized gene-targeted RXR alpha -/- embryos, which display an atrial-like ventricular phenotype with the development of heart failure and lethality at embryonic day 14.5. To quantitate the frequency and complexity of cardiac morphogenic defects, we now use microdissection and scanning electron microscopy to examine 107 wild-type, heterozygous, and homozygous embryos at embryonic day 13.5, 14.5, and 15.5. RXR alpha -/- embryos display complex defects, including ventricular septal, atrioventricular cushion, and conotruncal ridge defects, with double outlet right ventricle, aorticopulmonary window, and persistent truncus arteriosus. In addition, heterozygous RXR alpha embryos display a predisposition for trabecular and papillary muscle defects, ventricular septal defects, conotruncal ridge defects, atrioventricular cushion defects, and pulmonic stenosis. Lastly, we show that the intermediate anatomic phenotype displayed by heterozygous embryos is mirrored in the molecular marker MLC-2a. The intermediate phenotype of RXR alpha heterozygous embryos documents a gene dosage effect for RXR alpha in maintaining normal cardiac morphogenesis. In addition, some defects in RXR alpha mutant mice are phenocopies of human congenital heart defects, thereby suggesting that a relative deficiency in RXR alpha or molecules downstream in its signaling pathway may represent congenital heart disease-susceptibility genes.

Hepatocyte-specific mutation establishes retinoid X receptor alpha as a heterodimeric integrator of multiple physiological processes in the liver.

A large number of physiological processes in the adult liver are regulated by nuclear receptors that require heterodimerization with retinoid X receptors (RXRs). In this study, we have used cre-mediated recombination to disrupt the mouse RXRalpha gene specifically in hepatocytes. Although such mice are viable, molecular and biochemical parameters indicate that every one of the examined metabolic pathways in the liver (mediated by RXR heterodimerization with PPARalpha, CARbeta, PXR, LXR, and FXR) is compromised in the absence of RXRalpha. These data demonstrate the presence of a complex circuitry in which RXRalpha is integrated into a number of diverse physiological pathways as a common regulatory component of cholesterol, fatty acid, bile acid, steroid, and xenobiotic metabolism and homeostasis.

Compound mutants for retinoic acid receptor (RAR) beta and RAR alpha 1 reveal developmental functions for multiple RAR beta isoforms.

Mice with targeted disruptions in retinoic acid receptor genes have been generated to assess the role of nuclear receptors as transducers of the retinoid signal during vertebrate development. Mice with mutations that disrupt all isoforms of the RAR alpha, RAR beta and RAR gamma genes as well as for the individual RAR alpha 1, RAR beta 2 and RAR gamma 2 have been described. By breeding the RAR alpha 1 and RAR beta strains together we have generated double mutants which have striking phenotypes not discernible in mice homozygous for the individual mutations. Mice lacking both RAR alpha 1 and RAR beta died shortly after birth because of hypoxia, although individual RAR alpha 1 and RAR beta mutants were phenotypically normal. As previously observed in RAR compound mutants, histological examination of 18.5 dpc fetuses of RAR alpha 1 -/-beta-/- double mutants revealed a number of congenital malformations which in many respects were similar to those observed in fetuses of vitamin A-deficient mothers. The regions of congenital defects in RAR alpha 1 -/-beta-/- double mutants included the eye, the skull, the respiratory tract, the heart, the aortic arch-derived great vessels, and urogenital system. The penetrance of malformations in RAR alpha 1 -/-beta-/- mutants was greater than that in the reported RAR alpha 1 -/-beta 2-/- double mutants. Moreover, RAR alpha 1 -/-beta-/- mutants exhibited hypoplastic lungs and ossified fusion between basioccipital and exoccipital bones that were not reported in the RAR alpha 1 -/-beta2-/- animals, and displayed ectopic thymus and an unique defect in testis suggesting specific roles for RAR beta 1, 3 and/or 4 isoforms in these structures. The RAR alpha 1 single mutant animals as well as RAR alpha 1-/- beta-/- double mutant mice were susceptible to the teratogenic effects of RA, demonstrating that RAR alpha 1 and RAR beta isoforms singly or in combination do not play a major role in RA-induced craniofacial malformation and limb deformities.

Generation of a prostate epithelial cell-specific Cre transgenic mouse model for tissue-specific gene ablation.

To facilitate the elucidation of the genetic events that may play an important role in the development or tumorigenesis of the prostate gland, we have generated a transgenic mouse line with prostate-specific expression of Cre recombinase. This line, named PB-Cre4, carries the Cre gene under the control of a composite promoter, ARR2PB which is a derivative of the rat prostate-specific probasin (PB) promoter. Based on RT-PCR detection of Cre mRNA in PB-Cre4 mice or Cre-mediated activation of LacZ activity in PB-Cre4/R26R double transgenic mice, it is conclusively demonstrated that Cre expression is post-natal and prostatic epithelium-specific. Although the Cre recombination is detected in all lobes of the mouse prostate, there is a significant difference in expression levels between the lobes, being highest in the lateral lobe, followed by the ventral, and then the dorsal and anterior lobes. Besides the prostate gland, no other tissues of the adult PB-Cre4 mice demonstrate significant Cre expression, except for a few scattered areas in the gonads and the stroma of the seminal vesicle. By crossing the PB-Cre4 animals with floxed RXRalpha allelic mice, we demonstrate that mice, whose conventional knockout of this gene is lethal in embryogenesis, could be propagated with selective inactivation of RXRalpha in the prostate. Taken together, the results show that the PB-Cre4 mice have high levels of Cre expression and a high penetrance in the prostatic epithelium. The PB-Cre4 mice will be a useful resource for genetic-based studies on prostate development and prostatic disease.

The role of RXR-alpha in retinoic acid-induced cleft palate as assessed with the RXR-alpha knockout mouse.

Treatment of pregnant mice with retinoic acid (RA) in mid-gestation produces cleft palate and limb defects in the fetuses. RXR-alpha has been previously shown to mediate the teratogenic effects of RA in the limb. In this study, we show that RXR-alpha is also involved in retinoid-induced palatal clefting. Treatment of RXR-alpha knockout mice with a teratogenic dose of RA on gestation day 11 or 12 induces cleft palate at a lower frequency than that seen in wild-type animals.

Retinoic acid and retinoic acid receptors in development.

The vitamin A derivative retinoic acid (RA) and related compounds (retinoids) are utilized as signaling molecules in a diverse array of developmental and physiological regulatory processes, including many important in the developing and mature nervous system. Retinoids function by interaction with high affinity receptors of the nuclear receptor family, which also mediate the effects of steroid and thyroid hormones and which act in the nucleus as transcription factors. This review summarizes current knowledge of the molecular mechanisms of retinoid action, the complex role of retinoid receptors in a variety of hormonal signaling processes, and illustrates current efforts to more fully understand the biological functions of retinoid receptors through analysis of downstream gene regulatory networks and studies of mouse gene knockout systems.

The molecular genetics of retinoic acid receptors: cardiovascular and limb development.

The vitamin A metabolite retinoic acid (RA) is utilized as a signalling molecule in wide variety of developmental processes, defined by defects which occur after nutritional vitamin A deficiency or after exposure to excess vitamin A. We have initiated a genetic analysis of RA function through the establishment of lines of mice which carry germline mutations in the genes which encode retinoid receptors. Defects which result from developmental RA deficiency or excess have been recovered in embryos which are deficient in various combinations of retinoid receptors. In this chapter, our current understanding of the role of RA and retinoid receptors in cardiovascular and limb development are described, as for these our level of understanding is most advanced.

Molecular insights into cardiac development.

Recent discoveries have led to a greater appreciation of the diverse mechanisms that underlie cardiac morphogenesis. Genetic strategies (primarily gene targeting approaches in mice) have significantly broadened research in cardiovascular developmental biology by illuminating new pathways involved in heart development and by allowing the genetic evaluation of pathways that have previously been implicated in these events. Advances have also been made using biochemical and cell- and tissue-based approaches. This review summarizes the author's interpretation of current trends in the effort to understand the molecular basis of cardiac-development, with an emphasis on insights obtained from genetic models.

The RXRalpha gene functions in a non-cell-autonomous manner during mouse cardiac morphogenesis.

Germline mutation in mice of the retinoic acid receptor gene RXRalpha results in a proliferative failure of cardiomyocytes, which leads to an underdeveloped ventricular chamber and midgestation lethality. Mutation of the cell cycle regulator N-myc gene also leads to an apparently identical phenotype. In this study, we demonstrate by chimera analysis that the cardiomyocyte phenotype in RXRalpha-/- embryos is a non-cell-autonomous phenotype. In chimeric embryos made with embryonic stem cells lacking RXRalpha, cardiomyocytes deficient in RXRalpha develop normally and contribute to the ventricular chamber wall in a normal manner. Because the ventricular hypoplastic phenotype reemerges in highly chimeric embryos, we conclude that RXRalpha functions in a non-myocyte lineage of the heart to induce cardiomyocyte proliferation and accumulation, in a manner that is quantitatively sensitive. We further show that RXRalpha is not epistatic to N-myc, and that RXRalpha and N-myc regulate convergent obligate pathways of cardiomyocyte maturation.

Characterization of an autoregulated response element in the mouse retinoic acid receptor type beta gene.

A sequence that confers transcriptional responsiveness to retinoic acid was identified in the promoter of the mouse retinoic acid receptor (RAR) beta gene. This response element consists of a direct repeat of the sequence GTTCAC, separated by five nucleotides. Direct binding of the RAR to this sequence was demonstrated by gel retardation and immunoprecipitation assays. This element conferred retinoic acid responsiveness on heterologous promoters via all three subtypes of RAR yet failed to support transcriptional activation by the thyroid hormone, estrogen, glucocorticoid, or vitamin D receptors. Surprisingly, a high level of retinoic acid-dependent activation was seen in the absence of transfected RAR in 10 of 10 vertebrate cell lines, many functionally characterized previously as lacking endogenous receptor. This demonstrates an unusually high sensitivity of the retinoic acid response element to low levels of receptor and suggests expression of RAR in a wide variety of tissue types.

Retinoid-dependent pathways suppress myocardial cell hypertrophy.

Utilizing an in vitro model system of cardiac muscle cell hypertrophy, we have identified a retinoic acid (RA)-mediated pathway that suppresses the acquisition of specific features of the hypertrophic phenotype after exposure to the alpha-adrenergic receptor agonist phenylephrine. RA at physiological concentrations suppresses the increase in cell size and induction of a genetic marker for hypertrophy, the atrial natriuretic factor (ANF) gene. RA also suppresses endothelin 1 pathways for cardiac muscle cell hypertrophy, but it does not affect the increase in cell size and ANF expression induced by serum stimulation. A trans-activation analysis using a transient transfection assay reveals that neonatal rat ventricular myocardial cells express functional RA receptors of both the retinoic acid receptor and retinoid X receptor (RAR and RXR) subtypes. Using synthetic agonists of RA, which selectively bind to RXR or RAR, our data indicate that RAR/RXR heterodimers mediate suppression of alpha-adrenergic receptor-dependent hypertrophy. These results suggest the possibility that a pathway for suppression of hypertrophy may exist in vivo, which may have potential therapeutic value.

Normal development and growth of mice carrying a targeted disruption of the alpha 1 retinoic acid receptor gene.

Three unlinked genes encode receptors for retinoic acid (RAR alpha, -beta, and -gamma). Each gene expresses two major protein isoforms differing in the amino terminal A domain by alternative promoter use, fused to common exons encoding most of the receptor protein. The two RAR alpha transcripts (RAR alpha 1 and -alpha 2) are differentially expressed and evolutionarily conserved, as are the RAR beta and -gamma transcripts, suggesting that each isoform may have specific functions in the development of animals. To address the biological function of the alpha 1 receptor, we have disrupted the portion of the RAR alpha gene encoding this isoform by homologous recombination in mouse embryonic stem cells. Surprisingly, offspring homozygous for this mutation were viable and showed no apparently altered phenotype. RNA analysis confirmed that the RAR alpha 1 transcript was absent in homozygous tissues, and no evidence for a compensatory increase of RAR alpha 2 or of another RAR gene was obtained to account for the vitality of the mutant animals. These results clearly demonstrate that loss of RAR alpha 1 function does not disrupt embryonic development and argue for combinatorial or overlapping functions among the RAR isoforms.

Compartment-selective sensitivity of cardiovascular morphogenesis to combinations of retinoic acid receptor gene mutations.

Several aspects of normal cardiovascular development require signaling by the vitamin A metabolite retinoic acid. We have previously established germ-line mutations in mice in the genes that encode the RAR alpha 1, RAR beta, and RXR alpha retinoic acid receptors as a means of studying the function of these receptors in vivo. Although mutation of RXR alpha results in fetal ventricular defects, the RAR alpha 1 and RAR beta mutations are apparently nonphenotypic in the heart and elsewhere. In this study, we have established and analyzed combinations of these receptor gene mutations. Malformations of the ventricular chamber (chamber hypoplasia and muscular ventricular septal defects), conotruncus (double-outlet right ventricle, transposition, and membranous ventricular septal defects), aortic sac (persistent truncus arteriosus and aorticopulmonary window), and aortic arch-derived arteries were recovered in various combinations of the RAR alpha 1, RAR beta, and RXR alpha gene mutations. Depending on the combination of receptor mutations, selective defects were obtained in specific cardiovascular compartments, suggestive of differential expression or function of each receptor within domains of the developing heart.

Mouse embryos lacking RXR alpha are resistant to retinoic-acid-induced limb defects.

Embryonic exposure to the vitamin A metabolite retinoic acid (RA) causes malformations in numerous developing tissues, including the limbs, which serves as a model system of retinoic acid action. RA treatment of wild-type mouse embryos results in digit truncations and long bone reductions. These effects are mediated by products of the retinoic acid and retinoid X receptor genes (RARs and RXRs), members of the nuclear receptor family of ligand-dependent transcription factors. Mouse embryos homozygous for a mutation in the RXR alpha gene appear normal in limb development, although such embryos are phenotypically affected in other tissues. We now describe resistance to limb malformations normally induced by teratogenic RA exposure in the RXR alpha-/- background. RA treatments that cause limb defects in 100% of wild-type embryos fail to elicit malformations in RXR alpha homozygotes, implicating RXR alpha as a component in the teratogenic process in the limbs. Heterozygous embryos are intermediate in sensitivity to RA, suggesting the importance of RXR alpha gene dosage in limb teratogenesis. Expression of the RA-inducible gene RAR beta 2 was equivalent between wild-type and homozygous embryos after RA treatment. RA treatment also did not distinguish between wild-type and RXR alpha -/- embryos in the spatial expression of sonic hedgehog (Shh) and hoxd-12, two other genes implicated in limb development. However, the quantitative level of hoxd-12 expression was elevated in RXR alpha -/- embryos. These observations indicate that transcriptional processes which are inappropriately regulated in the mouse limb by exogenous RA require RXR alpha for their execution, and that specific teratogenic processes, as well as specific normal developmental processes under vitamin A control, occur through individual members of the RXR and RAR families.

Fate of the mammalian cardiac neural crest.

A subpopulation of neural crest termed the cardiac neural crest is required in avian embryos to initiate reorganization of the outflow tract of the developing cardiovascular system. In mammalian embryos, it has not been previously experimentally possible to study the long-term fate of this population, although there is strong inference that a similar population exists and is perturbed in a number of genetic and teratogenic contexts. We have employed a two-component genetic system based on Cre/lox recombination to label indelibly the entire mouse neural crest population at the time of its formation, and to detect it at any time thereafter. Labeled cells are detected throughout gestation and in postnatal stages in major tissues that are known or predicted to be derived from neural crest. Labeling is highly specific and highly efficient. In the region of the heart, neural-crest-derived cells surround the pharyngeal arch arteries from the time of their formation and undergo an altered distribution coincident with the reorganization of these vessels. Labeled cells populate the aorticopulmonary septum and conotruncal cushions prior to and during overt septation of the outflow tract, and surround the thymus and thyroid as these organs form. Neural-crest-derived mesenchymal cells are abundantly distributed in midgestation (E9.5-12.5), and adult derivatives of the third, fourth and sixth pharyngeal arch arteries retain a substantial contribution of labeled cells. However, the population of neural-crest-derived cells that infiltrates the conotruncus and which surrounds the noncardiac pharyngeal organs is either overgrown or selectively eliminated as development proceeds, resulting for these tissues in a modest to marginal contribution in late fetal and postnatal life.

A regulatory domain that directs lineage-specific expression of a skeletal matrix protein gene in the sea urchin embryo.

DNA sequences derived from the 5' region of a gene coding for the 50-kD skeletal matrix protein (SM50) of sea urchin embryo spicules were linked to the CAT reporter gene and injected into unfertilized eggs. CAT mRNA and enzyme were synthesized from these fusion constructs in embryos derived from these eggs, and in situ hybridization with a CAT antisense RNA probe demonstrated that expression is confined to skeletogenic mesenchyme cells. A mean of 5.5 of the 32-blastula-stage skeletogenic mesenchyme cells displayed CAT mRNA (range 1-15), a result consistent with earlier measurements indicating that incorporation of the exogenous injected DNA probably occurs in a single blastomere during early cleavage. In vitro mutagenesis and deletion experiments showed that CAT enzyme activity in the transgenic embryos is enhanced 34-fold by decreasing the number of SM50 amino acids at the amino-terminus of the fusion protein from 43 to 4. cis-regulatory sequences that are sufficient to promote lineage-specific spatial expression in the embryo are located between -440 and +120 with respect to the transcriptional initiation site.

A lineage-specific gene encoding a major matrix protein of the sea urchin embryo spicule. II. Structure of the gene and derived sequence of the protein.

A lambda gt11 cDNA clone isolated by use of a polyclonal antispicule matrix protein antiserum is shown in the accompanying paper [S. C. Benson, H. M. Sucov, L. Stephens, E. H. Davidson, and F. Wilt (1987) Dev. Biol. 120, 499-506] to encode a prominent 50-kDa spicule matrix protein (SM50). This clone was used to select homologous genomic recombinants, and the structure of the gene was determined. The SM50 gene occurs once per haploid genome. It contains a single intron located within the 35th codon. A unique transcription initiation site 110 nucleotide pairs prior to the translation start signal was mapped by primer extension. The mRNA is 1895 nucleotides in length, excluding the 3' poly(A) sequence, and contains a single open reading frame 450 codons in length. Though rare in whole embryo RNA the prevalence of the SM50 mRNA is calculated to be about 1% of the total mRNA in skeletogenic mesenchyme cells. The derived peptide sequence indicates a typical N-terminal signal peptide, and an N-linked glycosylation site near the C terminus. About 45% of the length of the protein is included in a domain composed of consecutive approximate repetitions of a 13-amino-acid element, the consensus sequence of which is Trp-Val-Gly-Asp-Asn-Gln-Ala-LeuTrp-Val-IleAsp-Asn-GlnPro+ ++-ValGlu. The protein also contains an internal domain unusually rich in proline residues and a very basic C-terminal region.

Peroxisome proliferator-activated receptor alpha-mediated pathways are altered in hepatocyte-specific retinoid X receptor alpha-deficient mice.

Retinoid x receptor alpha (RXRalpha) serves as an active partner of peroxisome proliferator-activated receptor (PPARalpha). In order to dissect the functional role of RXRalpha and PPARalpha in PPARalpha-mediated pathways, the hepatocyte RXRalpha-deficient mice have been challenged with physiological and pharmacological stresses, fasting and Wy14,643, respectively. The data demonstrate that RXRalpha and PPARalpha deficiency are different in several aspects. At the basal untreated level, RXRalpha deficiency resulted in marked induction of apolipoprotein A-I and C-III (apoA-I and apoC-III) mRNA levels and serum cholesterol and triglyceride levels, which was not found in PPARalpha-null mice. Fasting-induced PPARalpha activation was drastically prevented in the absence of hepatocyte RXRalpha. Wy14,643-mediated pleiotropic effects were also altered due to the absence of hepatocyte RXRalpha. Hepatocyte RXRalpha deficiency did not change the basal acyl-CoA oxidase, medium chain acyl-CoA dehydrogenase, and malic enzyme mRNA levels. However, the inducibility of those genes by Wy14,643 was markedly reduced in the mutant mouse livers. In contrast, the basal cytochrome P450 4A1, liver fatty acid-binding protein, and apoA-I and apoC-III mRNA levels were significantly altered in the mutant mouse livers, but the regulatory effect of Wy14,643 on expression of those genes remained the same. Wy14,643-induced hepatomegaly was partially inhibited in hepatocyte RXRalpha-deficient mice. Wy14,643-induced hepatocyte peroxisome proliferation was preserved in the absence of hepatocyte RXRalpha. These data suggested that in comparison to PPARalpha, hepatocyte RXRalpha has its unique role in lipid homeostasis and that the effect of RXRalpha, -beta, and -gamma is redundant in certain aspects.